drivers/timer/nrf_grtc_timer: Misc fixes

Misc fixes for the grtc timer driver:
* In non tickless mode:
  * The tick time would drift a bit with each interrupt
  * If something would cause a very significant delay
    in handling the tick interrupt the number of announcements
    would be incorrect
* Fortickless mode: The calculation of the next tick time
  in sys_clock_set_timeout() was incorrectly done,
  resulting in two spurious, too early, wakes of the kernel
  before each correct wake. This caused tests/kernel/context/
  to fail.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>